1. Field of the Invention
The present invention relates to a direct fuel injection type spark ignition internal combustion engine assisted in fuel evaporation by a squish flow.
2. Description of the Related Art
Direct fuel injection type spark ignition internal combustion engines are known, for example, in Japanese Patent Publication SHO 62-82222 and Japanese Utility Model Publication SHO 61-173728. In these engines, fuel is directly injected into a cylinder onto a surface of a piston. The fuel adhering to the surface of the piston is then evaporated by heat at the surface being assisted in fuel evaporation by a swirl, and then the evaporated fuel is ignited by a spark plug.
Further, though they are not a direct fuel injection type spark ignition engine, Japanese Utility Model Publication SHO 55-59136 and SHO 56-92717 disclose a squish portion for generating a squish flow, enlarged up to a position adjacent to a spark plug. The squish flow results from compression of the air and fuel between a piston and a cylinder head during movement of the piston toward the top dead center to force the air and fuel to flow toward the center of the cylinder. The publication SHO 55-59136 also discloses a groove formed at a squish portion defining surface. Further, Japanese Utility Model Publication SHO 58-142326 discloses a plug pocket portion, formed in a piston, for receiving a lower end portion of a spark plug therein when the piston is moved near a top dead center.
However, the above-described engines have some problems described below.
(a) Fuel evaporation at a surface needs an air flow to promote the evaporation. To generate such a flow in a cylinder, the direct fuel injection type engines use a swirl port, for example, a helical port. As a result, volumetric efficiency of the engine decreases at high engine speed operation due to flow resistance.
(b) Because flow speed of the swirl is comparatively slow, the flame propagation speed in a cylinder is slow. Therefore, especially in a high engine load operation where a fuel begins to be injected at an early time and thus the evaporated fuel is scattered to nearly all portions in the cylinder before it is ignited, the slow flame propagation makes inhibits the burning of an entire amount of the broadly scattered evaporated fuel. This decreases the engine power as well as increases a hydrocarbon amount in the exhaust gas. Further, especially in a low engine load operation where fuel is injected at a late time and thus there exists only a very short period of time before the ignition, insufficient evaporation of fuel due to the slow swirl flow is likely to cause failure in ignition.
(c) For the purpose of obtaining stable ignition in a low engine load operation, a cavity for holding evaporated fuel therein has to be formed in a piston with the cavity having a small diameter and deep depth. A plug gap portion of a spark plug has to be extended into the cavity. This means that the length of a portion of the spark plug exposed to the combustion chamber is long and the life of the spark plug is shortened. Further, the deep cavity is accompanied by an increase in compression height of the piston and thus increases the weight of the piston.
(d) Though the publication SHO 55-59136 and SHO 56-92717 disclose enlarged squish portions, it is impossible to combine such enlarged squish portions with the above-described swirl assist engine, because the squish flow can not flow into the deep cavity and can not evaporate the fuel adhering to the surface of the cavity. In addition, the squish flow may blow out the flame core formed in the vicinity of the spark plug, because the squish flow is much stronger than a swirl flow.